- NAME
- SYNOPSIS
- DESCRIPTION
- RETURN VALUES
- NOTES
- SEE ALSO
- HISTORY

# NAME

EVP_DigestSignInit, EVP_DigestSignUpdate, EVP_DigestSignFinal - EVP signing functions

# SYNOPSIS

     #include <openssl/evp.h>

     int EVP_DigestSignInit(EVP_MD_CTX *ctx, EVP_PKEY_CTX **pctx,
                            const EVP_MD *type, ENGINE *e, EVP_PKEY *pkey);
     int EVP_DigestSignUpdate(EVP_MD_CTX *ctx, const void *d, size_t cnt);
     int EVP_DigestSignFinal(EVP_MD_CTX *ctx, unsigned char *sig, size_t *siglen);

# DESCRIPTION

The EVP signature routines are a high level interface to digital signatures.

`EVP_DigestSignInit()` sets up signing context ctx to use digest type from ENGINE impl and private key pkey. ctx must be initialized with `EVP_MD_CTX_init()` before calling this function. If pctx is not NULL, the EVP_PKEY_CTX of the signing operation will be written to *pctx: this can be used to set alternative signing options. Note that any existing value in *pctx is overwritten. The EVP_PKEY_CTX value returned must not be freed directly by the application (it will be freed automatically when the EVP_MD_CTX is freed). The digest type may be NULL if the signing algorithm supports it.

`EVP_DigestSignUpdate()` hashes cnt bytes of data at d into the signature context ctx. This function can be called several times on the same ctx to include additional data. This function is currently implemented usig a macro.

`EVP_DigestSignFinal()` signs the data in ctx places the signature in sig. If sig is NULL then the maximum size of the output buffer is written to the siglen parameter. If sig is not NULL then before the call the siglen parameter should contain the length of the sig buffer, if the call is successful the signature is written to sig and the amount of data written to siglen.

# RETURN VALUES

`EVP_DigestSignInit()` `EVP_DigestSignUpdate()` and `EVP_DigestSignaFinal()` return 1 for success and 0 or a negative value for failure. In particular a return value of -2 indicates the operation is not supported by the public key algorithm.

The error codes can be obtained from ERR_get_error.

# NOTES

The EVP interface to digital signatures should almost always be used in preference to the low level interfaces. This is because the code then becomes transparent to the algorithm used and much more flexible.

In previous versions of OpenSSL there was a link between message digest types and public key algorithms. This meant that ``clone'' digests such as `EVP_dss1()` needed to be used to sign using SHA1 and DSA. This is no longer necessary and the use of clone digest is now discouraged.

For some key types and parameters the random number generator must be seeded or the operation will fail.

The call to `EVP_DigestSignFinal()` internally finalizes a copy of the digest context. This means that calls to `EVP_DigestSignUpdate()` and `EVP_DigestSignFinal()` can be called later to digest and sign additional data.

Since only a copy of the digest context is ever finalized the context must be cleaned up after use by calling `EVP_MD_CTX_cleanup()` or a memory leak will occur.

The use of `EVP_PKEY_size()` with these functions is discouraged because some signature operations may have a signature length which depends on the parameters set. As a result `EVP_PKEY_size()` would have to return a value which indicates the maximum possible signature for any set of parameters.

# SEE ALSO

EVP_DigestVerifyInit, EVP_DigestInit, err, evp, hmac, md2, md5, mdc2, ripemd, sha, dgst

# HISTORY

EVP_DigestSignInit(), `EVP_DigestSignUpdate()` and `EVP_DigestSignFinal()` were first added to OpenSSL 1.0.0.